In the field of telecommunications, efforts have recently been directed towards developing advanced direct sequence spread spectrum (DS-SS) telecommunications systems. One example of a DS-SS type system is a Code Division Multiple Access (CDMA) type system.
In a CDMA type system multiple users, each using a channel identified by a uniquely assigned digital pseudonoise (PN) code sequence, simultaneously communicate with the system while sharing the same wideband frequency spectrum. Channel identification through the uniquely assigned digital codes is achieved by using the unique PN code sequence to spread a digital information signal that is to be transmitted. The digital information signal may be a signal, such as the output of a digitized voice circuit, having a bit rate, for example, of from 8-13 kb/s or a data signal having a higher bit rate. The PN code sequence usually has a bit rate of several orders of magnitude greater than the information signal.
During spreading the digital signal bandwidth is spread through the frequency bandwidth of the PN code sequence. Spreading is achieved by multiplying the PN code sequence and information signal together in the time domain, to generate a spread signal that has a bit rate of the PN code sequence. The spread signal is then RF modulated and transmitted on a carrier frequency that may also carry transmissions of information signals for other system users, where the other information signals have been spread by PN code sequences unique to each of the other users. The PN code sequences may be uniquely identified by having a unique phase or a unique bit sequence. In certain systems, such as a system operating according to the Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) IS-95 cellular system standard, a transmission may be identified by two PN sequences. In IS-95 an individual base station is assigned a unique phase for a common system PN code sequence that spreads all forward link transmissions from base stations of that system. The unique phase identifies the base station. Each transmission from a base station is then also spread by a unique Walsh PN code sequence that identifies the particular base station channel on which the transmission is sent.
At the receiver, after carrier frequency demodulation, despreading is accomplished by generating a local replica of the transmitting user's assigned PN code(s) with a random-sequence generator in the receiver and then synchronizing the local PN code sequence(s) to the PN code sequence(s) that was superimposed on the incoming received signal in the transmitter. By removing the random sequence from the received signal and integrating it over a symbol period, a despread signal is obtained which ideally exactly represents the original digital information signal.
The process of synchronizing to a received signal is usually accomplished in two steps. The first step, called acquisition or detection, includes bringing the PN code sequences generated in the transmitter and receiver into coarse time alignment typically within one code chip interval. The second step involves fine synchronization to the signal and tracking to continuously maintain the best possible waveform alignment during reception by means of a feedback loop.
Because of the importance of synchronization many synchronization schemes have been proposed that utilize various types of detectors and decision strategies. A common feature of all synchronization schemes is that the received signal and the locally generated PN code sequence(s) are first correlated to determine the measure of similarity between the sequences. Secondly, the measure of similarity is compared to a threshold to decide if the signals are in synchronization. If there is no synchronization, the acquisition procedure provides a change in the phase of the locally generated PN code sequence and another correlation is attempted as a part of the signal search through the receiver's phase space.
The speed of signal acquisition and synchronization is generally an important performance factor in CDMA systems. For example, in an IS-95 system a mobile station must quickly search, acquire and synchronize to many different signals while maintaining communications with the system. The mobile station must initially acquire a pilot channel of the system upon power-up or entry into the system. As the mobile station moves through the system it must continually search, during ongoing communications, for stronger pilot channels of base stations located near the base station with which the mobile station is communicating. The pilot channels in IS-95 are transmitted by each base station using the same system PN code but with different offsets. The offsets allow the pilots to be distinguished from one another. All pilot channels in the IS-95 system use the Walsh code sequence of all ones to identify transmissions on the channel. The mobile station searches for pilot channels based on PN pilot channel phase information received from the system.
The mobile station must also search for phase varying multipath signals originally transmitted on a communications channel from a particular base station. Several multipath signals carrying the same information and on channels identified by the same system and Walsh PN code, but displaced in phase because of RF propagation effects, have to be searched so the strongest signals can be found and decoded. Also, during hand-offs between base stations utilizing the same carrier frequencies (soft hand-off), the mobile station must search for and acquire voice channels of target base stations while simultaneously maintaining communications on a voice channel with the current base station.
Initial acquisition or detection of a DS-SS signal may be accomplished by using the maximum likelihood acquisition method. In maximum likelihood acquisition, the received signal and the locally generated PN code sequence(s) are first correlated in the receiver to determine the measure of similarity between the signals. Secondly, the measure of similarity indicated by the correlated results is compared to a threshold to decide if the two signals are in coarse synchronization. The threshold may be determined a priori or may be an adaptive threshold, set according to the results of correlations with previous PN code phases. In the adaptive threshold method, the entire PN code space is searched and the PN code phase resulting in the maximum threshold is used to receive further communications.
In an IS-95 system, when the mobile station has detected acquisition of a pilot channel at a certain base station (or system) PN code phase, the mobile station attempts to decode a synchronization (SYNC) channel at the same PN code phase. This SYNC channel is spread by the base station PN code phase and a unique Walsh PN code sequence that identifies the SYNC channel transmissions. The SYNC channel frames transmitted on each SYNC channel from each base station are aligned with the pilot PN sequence of that base station, so correct detection and acquisition of the pilot channel allows the SYNC channel frame to be received and decoded. The SYNC channel frame includes a SYNC Channel Message that provides system parameters to the mobile station. The system parameters in the SYNC channel frame include the timing of the base station's pilot sequence with respect to the system timing and the base station's paging channel data rate. Once the mobile station has obtained information from the SYNC Channel Message, the mobile station adjusts its timing to correspond to the system's timing and begins monitoring the paging channel
If the process of synchronizing to the system, which includes acquiring the pilot channel and synchronizing to the SYNC channel, involves false detections of the pilot channel, significant penalties in time may result. In IS-95, if a mobile station falsely detects a pilot channel the mobile station attempts to transition to and decode the SYNC channel. The mobile station may spend up to 1 second attempting to decode the SYNC channel, after which the process of acquisition and synchronization will start again. The time spent attempting to decode the SYNC channel after false detection of a pilot channel is significant, considering that acquisition and synchronization times in the 2-to-3 second range are typical goals for mobile station manufacturers.
False detections of a pilot channel may be reduced by using a more accurate correlation process. Increasing the accuracy of the correlation process, however, may require an increase in the correlation time that itself significantly increases the time required for synchronization.